This invention relates to a punch used in connection with the profiling of the bottom end of a 2-piece drawn steel food container which is designed to be packed and then processed at high temperature and pressure in a retort. For the last 25 years, work has progressed on manufacturing drawn can bodies for food products. Drawn containers were made of materials such as aluminum and low temper steels in order to facilitate the drawing operation. In addition to this the containers usually had a height about equal to or less than the diameter of the container and the containers were fashioned in a single or at most two drawing operations. Similarly, the bottom profiles were bellow-like in that they moved in and out to accommodate pressure change during processing.
Only recently has it been possible to make multiple drawn 2-piece food containers which were fashioned from organically precoated tin free steel such that postcoating or post-treatment operations were not necessary. More particularly, a 24 oz. 404.times.307 tin free steel container was made in a two draw operation. (The can makers convention gives the diameter across the completed doubleseam in inches plus the number of sixteenths of an inch then the height in inches plus the number of sixteenths of an inch). Therefore, the foregoing container is 4 4/16" in diameter by 3 7/16" in height. It has long been desired to be able to make a 2-piece multiply drawn container whose height is appreciably greater than the diameter, using precoated starting material in a multiple draw process. It is also desired to make such a container in the popular 16 oz. 303.times.406 size or the 15 oz. 300.times.407 size or the 11 oz. 211.times.400 size.
The advantage of a drawn container is the elimination of the side seam and one of the double seamed ends present in a traditional 3-piece container. A 3-piece can body is made of a flat blank of material rolled into a cylinder and seamed along one side by welding, cementing or soldering. To this hollow cylindrical body is added a double seamed bottom closure. The cylindrical body may be precoated and the side seam area may be repaired by a stripe. The operations of flanging, side seaming, striping and double seaming are such that the quality of the container is dependent upon those manufacturing steps. In the past it has been the practice to use heavy gauge high strength metal to resist the processing stresses at the double seamed on ends for 3-piece containers e.g., 85 lbs per base box plate. More particularly, such containers include on their ends a deep countersink for strength and chuck clearance and even so those countersinks are subject to buckling during processing.
A 2-piece can with an integral bottom does not require a bottom end chuck countersink for double seaming, but a bottom recess is necessary in order to manufacture a 2-piece can with the same height and capacity as a conventional 3-piece can so that either may be interchangeably used in the same packing and processing line. The large capital investments in equipment for handling 3-piece cans in the packing plant cannot be merely written off. A 2-piece container which will physically resemble the 3-piece container is essential in order to permit continued use of the existing 3-piece equipment, e.g., labelling, runways, retort, etc.
Bottom profiling has been used to apply ribs, creases and the like to add rigidity to the bottom of the 2-piece can. The weakest area of a drawn 2-piece can is the bottom because the material thickness of the steel for 2-piece cans is about 65 lb plate and is less than a seamed on 3-piece end. With the thinner bottom of a 2-piece container strength required to give adequate buckle resistance is a function of the bottom configuration. With only moderate profiling, the pressurized 2-piece can bottom may distend and exceed the elastic limit of the metal. When that happens the can is unacceptable as it will rock about its distended bottom and appear to contain tainted or spoiled contents, as would result from the evolution of hydrogen gas. Consequently, a bottom profile is necessary to improve the performance of thin-two-piece cans. If the profile includes a relatively deep recess designed to have work hardened areas of metal, the elastic limit of the bottom metal is increased particularly in areas of high stress. The highly worked bottom is more rigid and is not bellow-like in configuration because the reverse drawing of the bottom profile increases the strength and decreases the container volume.
Each hermetically sealed container must be retorted to prevent bacterial growth and spoilage which will generate metabolic products such as organic acids and carbon dioxide; the latter tending to inflate the sealed container causing it to bulge or become unseamed. In order to have commercial sterility (safety) the food must be heated to a state which renders it free of viable forms of micro-organisms which are there or which would reproduce in the future under normal storage conditions. Cans with hermetically sealed contents heated above their boiling point and then cooled subjects the bottom profile to internal pressure and then external pressure. Similarly, certain groups of high acid foods need not be retort processed since these acidic foods are hot packed. That is to say that, they are heated to near the boiling point and then packed in the container. Even hot packing places considerable pressure stress on the container. The bottom and top ends get more of the pressure load. The top end can be made of heavier plate whereas the bottom end of a 2-piece container is of the minimum gauge from which the container body may be drawn so that the weight of the container and the thickness of the side walls are kept to a minimum. Spoiled contents will then tend to first force outward the seamed on top and simplifing the process of checking the packed cans.
It is very difficult to form such bottom profiles by reverse drawing without highly stressing the metal in the body sidewall and bottom since there is considerable stretching required in order to generate a bottom recess deep enough to provide the height to volume ratio of a similar 3-piece container. Moreover, DR9 plate is double reduced and any work hardening causes that highly stressed metal to quickly reach its ultimate tensile strength.
The preferred embodiment of and method for producing a 2-piece container of a minimum amount of the high temper double reduced steel involves one to three concurrent drawing operations which may take place in a press as disclosed in U.S. Pat. No. 4,262,510 (incorporated herein by reference) which is assigned to the same Company as the present invention. To make a triple drawn can by drawing and ironing, the first operation blanks and then forms a sheet of precoated material into a shallow cup with a diameter in excess of its height. In the second operation the diameter is reduced and the height increased so that they are about equal in the container formed. The last operation reduces the diameter still further such that the container achieves its final configuration. During this last operation the bottom profile is reverse drawn into the container. After that reverse draw the bottom profile has all the essentials of the final desired profile including, for example, a domed central recess. The drawn can bearing that preferred profile shape has an annular flat outer circumferential portion extending from the lowermost corner of the sidewall and bottom inwardly to an upwardly and inwardly slightly inclined annular wall concentrically located relative to the central longitudinal axis of the can. The annular wall tips slightly toward the axis of the can and extends inwardly towards the outward circumferential portion forming the boundary of the central domed recess. The dome is essentially parallel to the bottom plane defined by the aforesaid flat circumferential portion radially outward of the central domed recess. The central recess is essentially parallel to the bottom plane defined by the aforesaid flat circumferential portion in that the same forms a relatively shallow and long-radius dome when formed by doming inwardly along the axis of the container body.
During the drawing and ironing operations used to form and reform, high clamping forces are needed. Conventional drawing of cups, box shapes, or irregular panels causes wrinkles to occur at the outer region of the blank and to prevent wrinkling in the first draw die, blankholders are used to clamp. The clamping force will vary with variation in sheet-metal thickness. Normal variations in sheet-metal thickness can cause problems. Thinner sheet thickness reduces the clamping force resulting in wrinkles. Thicker sheet could, on the other hand, cause tearing due to excessive clamping force. The clamping force causes the sheet metal to thicken rather than wrinkle under the squeezing loads of clamping. Because too much force will cause either tearing and too little force wrinkling of the sheet metal, the clamp force is critical. Constant pressure is desirable for clamping, particularly for the start of the drawing process. Actual clamping force requirements are determined by trial and error as the draw die is tried out. Clamping restrains the movement of metal through the die and imparts stretching forces to the container being formed.
The metal working of the bottom panel which provides the central recess is done in such a manner that the amount of material in the initial blank is not increased as much as the recess requires. More particularly, the central recess is formed substantially from stretching and pulling the metal from the side wall into the bottom panel. This is beneficial from the standpoint of economical material usage and is important from the standpoint of work hardening primarily the bottom and side wall to improve their yield strength, but same is detrimental from the point of view of metal flow during the reverse drawing of the bottom recess. That is to say that, the clamping of the metal to prevent wrinkling during drawing is about the flange and so the force applied to the bottom during the reverse drawing of the domed recess must pull metal from the entire sidewall into the bottom profile. During reverse drawing sidewall and bottom metal are stretched over a domed punch nose causing the sheet to thin at the crown of the dome. To assure metal stretching without wrinkles, the clamping force must be high enough to prevent or greatly retard uncontrolled movement along the punch.
If ironing is added to the process the problem of reverse redraw stress is increased since the sidewall is clamped not way up at the flange but along the sidewall closer to the area from which metal must be pulled into the bottom profile. Ironing is most desirable from a manufacturing economics and performance standpoint since less metal is required to make the container body and because the finished can body has been work hardened to a higher level. The high temper low stretch double reduced plate cannot be stretched more than 5 to 10% without increased risk of fracture. In particular, stretching of larger percentage amounts causes tearing of the sidewall or breaking of the dome in the bottom profile. With ironing stretch of 16 to 18% has been measured in a typical 303.times.406 container.
With low temper materials capable of stretching or with unironed sidewalls or with a combination of those, the bottom profile can usually be formed without tearing or breaking the can during the reverse draw of the domed recess. The economics of commercial can production necessitate the use of high temper (DR 8 or 9 steel), thin guage (65 lb) plate, ironed in order to prove adequate strength and light weight in a 2-piece container. Thus, a way in which commercially practical containers could be made with the requisite bottom profiling necessary for 3-piece interchangeability and to overcome the end stresses of packing and processing had to be found.